Optical fiber for long-haul communications has reached a remarkable state of perfection. For instance, single mode fibers having loss of about 0.20 db/km are rountinely being produced. Nevertheless, there is still great interest in further reducing the signal loss, since even a reduction as small as 0.01 db/km can translate into a significant increase in the permitted distance between repeaters. This in turn can translate into a significant difference in system cost, especially for transmission systems such as transoceanic fiber optic systems that, by necessity, have to employ highly complex (and thus costly) repeaters.
A related aspect is the need to reduce system cost through reduction of fiber cost. For instance, if the length of fiber drawn from a preform can be increased substantially, without proportional increase in the cost of producing the preform, then a substantial reduction in fiber cost will generally result.
As is well known, the optically active part of an optical fiber (i.e., the core and the surrounding "deposited" cladding) generally consists of glass that is synthesized in a glass-forming reaction and deposited on a substrate. See, for instance, U.S. Pat. No. 4,691,990, which is incorporated herein by reference.
There are currently two categories of methods for producing the optically active portion of optical fiber in commercial use. One category comprises the so-called outside processes (OVD and VAD), and the other the so-called "inside" processes (MCVD and PCVD). This application pertains primarily to the fiber produced by the latter processes, although the invention can also be practiced with at least some variants of outside processes.
As is well known, in the inside processes the glass-forming reaction is caused to take place inside a silica-based tube (generally referred to as the "substrate" tube) and the resulting glassy reaction product is caused to be deposited on the inside wall of the substrate tube.
A major component of the cost of producing optical fiber is the cost of depositing the core and deposited cladding material, collectively to be referred to as the "deposited" glass. Thus, it would be desirable to be able to draw more fiber from a given preform without, at the same time, proportionately increasing the deposition time.
Artisans know some methods that can be used to decrease the percentage of deposited glass in a fiber, without incurring a substantial loss penalty. The discussion will be in terms of a widely used, commercially available single mode fiber design which is schematically shown in FIG. 1.
The inner portion of the fiber (consisting of in situ-produced deposited silica-based glass) consists of core 10 (effective refractive index n.sub.c, effective diameter d), surrounded by first cladding region 11 (effective refractive index n.sub.1, effective outer diameter D). Typically, n.sub.c &gt;n.sub.o, and n.sub.1 &lt;n.sub.o, where n.sub.o is the refractive index of nominally pure vitreous silica. As will be readily appreciated, refractive indices are compared at a given wavelength, e.g., the intended operating wavelength of the fiber.
FIG. 1 also shows second cladding region 12 (outer diameter D.sub.o, refractive index n.sub.2 =n.sub.o) that generally consists of glass derived from the substrate tube and that surrounds the inner portion of the fiber. Typically, for low loss fibers D/d has to be at least about 5, due in part to the relatively high loss of available silica glass tubes but, typically more importantly, due to unacceptably high macrobending losses that could occur (for designs having relative large index differences) for D/d.ltorsim.5.0. It is to be noted that macrobending losses can occur due to the existence of the index step between the first and second cladding regions, and could occur even if the material of the second cladding region had very low loss. The fiber diameter (D.sub.o) is such that D.sub.o /d is about 15, D/d is between about 5.5 and about 6.5, resulting in a fiber in which typically about 19% of the total volume is deposited glass.
More fiber per preform could be obtained if more glass were deposited and a thicker substrate tube were used. However, this generally is not feasible when an inside deposition process is used, since a thick substrate tube typically would have too much thermal impedance and make it difficult to sustain the glass-forming reaction inside the tube with a conventional outside heat source and to subsequently collapse the tube to a rod. Furthermore, the time required for deposition would increase, limiting any potential improvement in economics.
In order to overcome this limitation it has been proposed that an appropriately scaled-up amount of deposited glass be synthesized inside a standard silica tube, the tube be collapsed, and the thus produced rod-like glass body be overclad by shrinking a silica tube around the body. Exemplarily, for D/d of 5.5 and D.sub.o /d of about 15, in the so-called "rod-in-tube" process about 13% of the total fiber would be deposited glass, 40% would be derived from the substrate tube, and about 47% from the overclad tube. If a constant value of D/d is maintained by an increase in the amount of material deposited onto the substrate tube, the use of an overclad (as in the rod-in-tube process, or produced by any other overcladding technique) generally does not materially reduce the percentage of deposited glass in a fiber. However, it can lead to somewhat reduced fiber costs due to decreased set-up time per unit length of fiber, and has the benefits associated with longer continuous lengths of fiber from a larger preform, including the possibility of using higher draw speeds.
The above cited '990 patent discloses an approach that can be used to reduce the percentage of deposited glass in a silica-based fiber. FIG. 2 schematically depicts an exemplary index profile of fiber according to the '990 patent, wherein n.sub.2 &lt;n.sub.o (typically n.sub.2 .about.n.sub.1). Due to favorable macro-bending characteristics of such a fiber design, it is possible to reduce D/d to a value below the 5.5-6.5 range without incurring significant loss penalties. If the material of the second cladding region (12) has relatively low loss, it is possible to reduce D/d to about 3. Low loss down-doped silica tubes are becoming available now. Exemplarily, for D/d=3 and D.sub.o /d=15, only 4% of the fiber would be deposited glass, the remainder being tube-derived glass. However, the design typically would not have all the propagation advantages of the depressed cladding design of FIG. 1, and would require low draw speeds to minimize the stress in the core.
In view of the commercial significance of improved methods of optical fiber manufacture, a method that can increase the amount of fiber that can be drawn from a preform, and/or that can result in fiber having lower loss, would be of great interest. This application discloses such a method, as well as the novel optical fibers produced by the inventive method.